1. Field of Art
This invention relates generally to electromagnetic actuators, and especially to thrust actuators for magnetic bearing systems. Improvements in thrust bearing specific force capacity allow for smaller, lighter stators and enable operation at higher speeds.
2. Description of Related Art
Active magnetic bearing (AMB) systems usually have two opposing thrust bearings, one on either side of a thrust disk that is mounted on the shaft. By controlling the current in each of the bearings, the net force on the thrust disk can be controlled in either direction to stabilize the shaft assembly in the axial direction against both static and dynamic forces.
Thrust bearings for AMB systems generally consist of three major components. The stator, which is a stationary cylindrical component made of magnetic material, provides a flux path and houses the coil. The coil, which is a toroidal winding of electrically conductive wire, provides the magnetic flux when power is applied. The thrust disk, which is a flat cylindrical component rigidly attached to the shaft, is composed of a magnetic material that completes the flux path.
The stator and thrust disk are separated by an air gap. When a current passes through the coil, it creates a magnetic flux that travels through the stator and thrust disk, creating a net force that pulls the thrust disk (and therefore the entire shaft assembly) toward the stator. Viewing the thrust bearing in an axisymmetric cross section, the flux path is a closed loop that circles the coil cavity. The path travels through the horseshoe shaped cross section of the stator, across the air gap, radially through the thrust disk, traverses the air gap once more, and back to the stator.
The force generated by the thrust actuator depends on several factors, including the magnetic and structural properties of the stator and thrust disk, the coil current density, the size of the air gap, and the geometry of both the stator and thrust disk.
The first step in sizing a thrust bearing actuator is determining the desired maximum force capacity, which is the force developed when the magnetic material in the stator is saturated with flux and it coincides with the maximum current density in the coil. Using traditional methods, the pole area of the stator is usually determined based on a one-dimensional equation that relates pole area (Apole), saturation flux density (Bsat), and force at saturation (Fsat):
                              F          sat                =                                            A              pole                        ·                          B              sat              2                                            μ            0                                              (        1        )            Using this relation, the dimensions of the inner pole, outer pole, and back face of the stator are determined. The coil area (Acoil) is generally made large enough to magnetically saturate the stator material at the maximum current density (Jsat) using the following equation, where g is the air gap:
                              A          coil                =                              2            ⁢                          g              ·                              B                sat                                                                        μ              0                        ·                          J              sat                                                          (        2        )            Using this coil area, the coil dimensions are selected and generally made such that the cross-sectional area is square or rectangular. The final dimension selected is the thrust disk thickness, which usually matches the thickness of the stator back face.
In general, the configurations described in prior art are not optimal due to flux leakage and inefficient use of stator magnetic material. The objective in producing a particular configuration is to achieve uniform saturation throughout the entire stator. Traditional configurations, however, usually saturate near the transition between the inner pole and the back face even if the coil area and dimensions are chosen properly. Since not all of the stator material is fully saturated, some volume is effectively being wasted. The problem is compounded by the fact that some of the thrust bearing volume could be utilized by making the coil larger, which would in turn provide more available capacity for a given current density. Accordingly, it is an objective of this invention to parameterize the geometry in such a way as to allow for the most efficient use of volume to create the highest thrust bearing capacity.
FIG. 1 shows an axisymmetric cross section that is representative of thrust bearings in the prior art, such as in U.S. Pat. No. 5,525,848. The three major components are the thrust disk 100, the coil cavity 101, and the thrust stator, which has an inner pole 102, back face 103, and outer pole 104. The stator and thrust disk are separated by an air gap 105 and both have an axis of symmetry 106. In general, this configuration is employed when the thrust disk diameter can be made as large as the outer diameter of the thrust stator.
In some applications, such a thrust disk would lead to failure of the disk material at high rotational speeds due to excessive stress at the thrust disk hub. In these cases, a “folded” thrust disk or L-shaped configuration is employed as in U.S. Pat. No. 6,008,558.
In some other cases, such as when axial space is very constrained, an “E-core” configuration is used, such as in U.S. Pat. No. 4,920,291. This design features two adjacent rectangular coil cavities, resulting in three poles, much like the letter E in cross section. The “E-core” design, however, requires a large stator diameter and thrust disk diameter. Therefore, it can only be used when the rotational speed is low.
In most cases, the thrust stator geometry is similar to that pictured in FIG. 1, or if not, it usually suffers from the same drawbacks. The major problem with these designs is that a specific part of the stator saturates before the rest of the stator. The prematurely saturated region 107 is shown in FIG. 1 for the representative geometry. The presence of saturation at the transition between the inner pole 102 and the back face 103 indicates that the cross-sectional area normal to the flux path is not large enough to accommodate the flux density at this point. This results in a sub-optimal design in which not all of the stator material is used efficiently, and hence the force capacity is not maximized.
Even if the dimensions of the rectangular region are optimized, the problem is not rectified. It is an inherent property of this parameterization that results in an inefficient thrust actuator design.